The invention relates to a method of firing an oven for heating material therein by gas burners or oil burners. In many such ovens flames extend from the burners into and partly through a more or less empty heating chamber around the material to be heated and the temperature of the oven is regulated or controlled by adjusting the rate at which the burners supply the fluid fuel to the oven.
In such systems and particularly in industrial ovens the streams of fluid fuel are readjusted from time to time, in response to changing temperatures in the ovens, either by way of steady regulation of the rate of supply of fluid fuel or by periodically resetting such rate to a high and a low or zero magnitude.
In the steady regulation systems the burners can be adjusted to various degrees of opening thereof between wide open and closed conditions. Such regulation has the advantage that it can respond finely to the heat demand of the oven. However, it has the following disadvantages. When the heat demand of the oven is small, the flames become short. As a result, temperatures in different parts of the oven become unequal, as the flames and the circulating flue gas heated thereby do not reach all parts of the oven at equal temperatures. Radiation of heat from the flames can also become unequal in different parts of an oven. These difficulties are encountered particularly where the material to be heated extends parallel to the flames, as is the case for example in ovens wherein ingots are transported from one to the other end of the oven and are heated by burners inserted in the sidewalls of the oven and emitting flames transversely of the direction of travel of the ingots. When the heat load is small the length of the flames is correspondingly small and the ends of the ingots, adjacent the sidewalls, are heated more strongly than the middle portions of the ingots, in the central part of the oven.
This disadvantage is avoided by controlling the burners in a continuous cycle between two predetermined burner settings, that is between open and closed settings or between wide open and narrowly opened settings. The disadvantage of the steady regulation is avoided by the opening and closing control, and is partly avoided by the wide and narrow setting control, being avoided in the latter case so much better the smaller the narrow opening is made. However, these two-point controls have disadvantages as they change the burner positions suddenly. For one thing, the burner setting members are subject to very considerable wear and tear under this kind of control; so particularly in systems using hot air. Additionally the supply of fuel and air to the oven often tends to lag behind the sudden changes of burner settings, thereby causing operating difficulties in the oven, such as the occurence of minima and maxima of pressures in the fluid supply. These in turn tend to interfere with maintenance of the desired proportion of fuel and air, at the times of changing the settings, thereby to cause changing composition of the oven atmosphere, and in many cases to cause trouble incident to the reigniting of the burners. Additionally, it becomes difficult to maintain suitable pressure in the oven chamber upon the sudden changes of rate of flow of fluid fuel. These difficulties can be overcome only in some limited cases and by special and expensive means. For example when all burners of an oven are simultaneously closed in the open-closed control system, maintenance of desired pressure in the oven chamber is possible only in an oven hermetically closed, particularly on top, like a bell jar. This is impossible in many cases. Therefore, undesired drafts occur upon the sudden change of flow rate of fuel, leading to unequal temperatures and poor composition of oven atmospheres. In addition dangers of sudden breaking out of burning flue gas may be encountered with ovens so controlled, particularly with those built for high heating effect and wherein oven doors must be opened from time to time during the operation. Recuperator ovens also are excessively loaded by large and sudden changes of amounts of fuel supplied to them.